Jonathan Moffett uses 3- and 5-bass drum setups. That's what I learned from a YouTube video I've seen yesterday. A post on DW (about tuning approaches) got me look that one up, I had never seen/heard that man before.
The bass drums he's using all differ in size and thus, sound. He's trying to play 'melodies' on the bass drums and switches between bass pedals a lot. His 5-bass drum set has a 20, 22, 24, 26, 28 bass drum!
The Rev from Avenged Sevenfold had 3 kicks, but used 2.
Alex van Halen actually had 8 bass drums at one point, it was 4, with another 4 in front like woofers. Either Peter Kriss or Eric Carr from Kiss ( I think PK) did the same.
I think as a rule of thumb, there is a correlation btween how many kicks you can squeeze on stage and how badass you are. Therefore, just make a kit entirely out of bass drums and be the baddest mofo on the planet!!
It must be a pain in the ass playing out with 2 kicks. Luckily i was in a hardcore band and big kits weren't popular. More time setting up, more space taken up in the van, more space taken up on stage, more time tuning. I guess its worth it if your in a metal band though. Seeing Slayer with a single kick would be weird.